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I just realized that a chunk of my message did not post and caused my point to get confused.
What I was referring to when I said "successfully sued" was that you can be sued and loose if you are in charge of a group, even if you have liability releases.
Being sued as a professional is always a possibility. Dot your i's and j's, don't be negligent, and you should be fine.
When on vacation, I show my AOW and Nitrox card (if Nitrox is available). I do this because showing your DM card advertises that you probably have high dollar liability insurance.